In a nuclear power plant, primary cooling water that has been heated in a nuclear reactor is fed to a steam generator, secondary cooling water is heated in the steam generator with the heat of the primary cooling water to generate secondary steam, and the secondary steam is supplied to a steam turbine to rotate the steam turbine, thereby driving an electric generator.
FIG. 9 is a diagram showing, in outline, the configuration of a steam generator that is used in a nuclear power plant. As shown in FIG. 9, a steam generator 1 is provided with a solid outer cylinder 2, a tube plate 3 that is formed integrally with the outer cylinder 2, and a number of inverted U-shaped heat transfer tubes 7 both ends of which are inserted in and connected to the tube plate 3. A number of heat transfer tubes 7 having several bend radii form a heat transfer tube bundle, and the heat transfer tube bundle is surrounded by an inner cylinder 4 and is supported horizontally by a number of tube support plates 5.
A number of through-holes for allowing the heat transfer tube 7 to pass through are formed in the tube support plates 5. The tube support plates 5 are used for preventing vibration of the heat transfer tubes 7 and for maintaining the mutual spacing between the heat transfer tubes 7. In other words, since a number of heat transfer tubes 7 pass through the tube support plate 5, the mutual spacing between the heat transfer tubes 7 can be maintained, thus preventing vibration of the heat transfer tubes 7, and, furthermore, even when a lateral load (a load exerted in the horizontal direction) is exerted due to an earthquake etc., this lateral load is borne by the tube support plate 5.
In the steam generator having such a configuration, high-temperature heating fluid 6 flows into a first water chamber 8, and this heating fluid 6 flows through the heat transfer tubes 7, provides heat to be cooled, and then, flows out from a second water chamber 9. The heating fluid 6 is, for example, a nuclear reactor coolant. In addition, feedwater 31 that has entered by flowing through a feedwater nozzle 30 flows down through the space between the inner cylinder 4 and the outer cylinder 2, turns above the tube plate 3, and flows upwards along the heat transfer tubes 7. While flowing upwards, the feedwater 31 absorbs heat from the heating fluid 6 through heat exchange, is heated, is boiled, and becomes steam. This steam 33 flows out from a steam nozzle 34 and is directed to, for example, a steam turbine.
FIG. 10 is a plan view of the tube support plate 5 taken from above the steam generator 1. As shown in FIG. 10, wedges 50 are provided on the outer periphery of the tube support plate 5 at prescribed intervals. When the tube support plate 5 is arranged in the steam generator, these wedges 50 fill the gap between the inner cylinder 4 and the tube support plate 5.
In addition, a number of (for example, a few thousand) through-holes 100 (see FIG. 11) for allowing the heat transfer tubes 7 to pass through are formed in the central portion (cross-hatched portion in the figure) of the tube support plate 5. As shown in FIG. 11, the through-holes 100 have, for example, a hexagonal shape, and protrusions 100a, 100b, and 100c that protrude towards the center of the through hole 100 are respectively formed on, among the six sides 101, 102, 103, 104, 105, and 106 that form the hexagonal shape, the alternate sides 102, 104, and 106. In FIG. 11, although only four through-holes 100 are illustrated, in practice, such through-holes 100 are arranged on the tube support plate 5 so as to be aligned vertically and horizontally in a grid pattern. In a state where the heat transfer tubes 7 pass through such through-holes 100, respectively, a gap is provided between the outer peripheral surface of the heat transfer tubes 7 and the inner peripheral surface of the through-holes 100, and this gap provides a flow path for the secondary cooling water and the steam. The through-holes 100 have, in addition to a function of supporting the heat transfer tubes 7, a flowpath function for allowing the secondary cooling water and the steam that flow upwards in the inner cylinder 4 to flow therethrough.
When the steam generator 1 experiences a large lateral load due to an earthquake etc., the lateral load is also propagated to the tube support plate 5. At this time, if the in-plane compressive strength of the tube support plate 5 is not sufficient, there is a risk in that the through-holes 100 will be crushed to contact with the heat transfer tubes 7, and the heat transfer tubes 7 will be deformed (damaged). In order to prevent the occurrence of such a situation, the tube support plate 5 is required to have a certain in-plane compressive strength.
Conventionally, an evaluation of the in-plane compressive strength of such a tube support plate 5 is performed by forming a small-scale test piece having the same structure as the tube support plate 5, gradually applying a lateral load to this test piece, and reading out the value of the lateral load (hereinafter referred to as “stick load”) at which the through hole 100 that is formed in the tube support plate 5 is crushed and makes contact with the heat transfer tubes 7. The reason for performing the evaluation using a test piece in this way is that, since the actual dimension of the tube support plate 5 is at least 3 m in diameter and can be as large as about 6 m in diameter, test facilities, test conditions, and so forth are restricted in several ways, and it is difficult to perform the in-plane compressive strength evaluation with the actual plate.
By performing the evaluation of the in-plane compressive strength using a test piece in this way, it is possible to estimate, when a lateral load is exerted on the actual plate, how the tube support plate 5 will deform and the position in the tube support plate where the most severe damage will be caused.